LIKED
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ACTING
The acting is magnificent; the leads can do more with a five-second look than many actors do with 16 episodes. The supporting cast is also quite good.
CHARACTERS
Him - he's an arrogant genius, a true genius. He's not Do-san; he's not just good at programming and weak elsewhere. His genius comes through in his ability to analyze business trends, in art, in philosophy, in predicting people's behavior. He's a slightly saner, slightly nicer Hannibal Lecter. And coupled with that genius is an absolute arrogant belief in himself and his superiority over others, it's both his greatest weakness and his greatest strength. Couple all of that with an iron will and you have a truly formidable and often downright frightening hero. Numerous times I found myself shaking my head at his behavior but fully understanding it.
Her - The extent to which she falls for him is frightening. She will forgive nearly anything he does and has an unshakeable trust in both his ability and his decisions, even though he makes numerous bad decisions throughout the show. You'd expect her scary love combined with his super arrogant, ultra genius to lead to an unequal relationship and there are certainly moments of inequality but overall this is one of the more equal screen partnerships you'll ever see. The directors walked a tightrope here and, for the most part, kept upright.
CHEMISTRY
Romance - these two... I'm not a big believer in chemistry as I often disagree with folks who say characters have it or don't have it but these two? This is the definition of chemistry. This show has some of the finest hugs, kisses, and conversations you'll ever see onscreen. There are several scenes between them that I've watched over and over and over. They are sooo gooood together!
MUSIC
I rarely notice soundtracks but as you're watching the captioned lyrics crawling across your screen - they become an element of the story, reinforcing the visuals. And catchy? I literally can't get Zhou Shen's fireworks out of my head.
OTHER
Cinematography - it's outstanding. It doesn't overpower the drama but every scene perfectly sets the mood.
The Plot - I was immediately hooked and despite some issues (we'll get to those) it worked for me.

DIDN'T LIKE
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First - let's be clear - I loved this drama so these are quibbles for me but for some folks they might be more problematic

Lack of growth - Our lead will make some mistakes. And he has some character flaws. And that's okay! But I didn't feel that he necessarily learned from those mistakes or fixed his flaws. There wasn't a lot of observable growth.

Time Jumps - The plot hooks you right from the beginning but... but then we take a LOOONG trip down memory lane to how their relationship started. Initially, I was frustrated as I wanted to get back to the present but this part of the story is interesting so once I got hooked on THAT part of the story I was okay. But it took a few episodes.

Love Denial - one of my pet peeves is when a character decides that their OTP would be better off without them so they sabotage their relationship, often in a way that is nearly guaranteed to turn their partner into a soulless, sobbing husk of a human. Unfortunately, there's a lot more of that going on here than I like to see. Having said that - it worked for me as much as this trope can ever work for me - I could understand the motivation even though I didn't agree with the decision.

School Setting - I'm not generally a fan of school settings and a good chunk of this show is set in school. But without being too spoilery, the ostensible school setting is better understood and more closely akin to a work setting.

Plot Holes - there are some glaring plot issues if you stare too long or look too closely. They didn't bother me too much but the story would have been better if they'd fixed them.

SOME PEOPLE MIGHT NOT LIKE

Work Setting - this entire drama is set around a series of work challenges. I enjoyed this but I've heard that some people are not fond of this kind of drama.